Depending on the type of vehicle, an engine brake may be required to operate in as low an engine speed range as possible. For example, the use of a one-way clutch, as disclosed in Japanese Patent Publication 2005-207515 of Aug. 4, 2005 (“Patent Document 1”), is one method that would allow an engine brake to operate in as low an engine speed range as possible.
Specifically, as disclosed in Patent Document 1, when an outer clutch member rotates faster than an inner clutch member, a one-way clutch, which connects the outer clutch member and the inner clutch member, may be provided in order to transmit a back torque to the crankshaft. According to this setup, as described in Patent Document 1, even after the rotational speed of the crankshaft is decreased and a centrifugal clutch is disengaged, the engine brake works effectively owing to the back torque transmitted to the crank shaft by the one-way clutch.
The present invention has been devised in light of the above-described circumstances, and it is an object thereof to provide a clutch structure which is constructed without a one-way clutch, and which is not disengaged until the input side rotational speed becomes relatively low in order to comply with the requirement that the engine brake works in as low an engine speed range as possible.